Process for brazing of aluminum alloys and a flux

ABSTRACT

A process for brazing of aluminium magnesium alloys is described applying a flux which comprises KAlF4 or CsAlF4 or both as major constituent. The flux further comprises at least one alkaline or alkaline earth metal compound selected from the group consisting of KAlF4, CsAlF4, Li3AlF6, CaF2, CaCO3, MgF2, MgCO3, SrF2, SrCO3, BaF2, and BaCO3. Preferably the flux comprises or consists of KAlF4, CsAlF4, and Li3AlF6 and optionally contains also BaF2.

This application claims priority to European application No. 14164856.8filed Apr. 16, 2014, the whole content of this application beingincorporated herein by reference for all purposes.

The present invention concerns a process for brazing of aluminum alloyscontaining magnesium, and a suitable flux.

It is well known in the art that brazing of aluminum parts can beperformed utilizing fluxes based on alkali metal fluoroaluminates.Fluxes of this type are generally considered to be noncorrosive. Forexample, a flux is described in U.S. Pat. No. 4,579,605 which comprises5% to 95% by weight of K₂AlF₅ or its hydrate; the remainder is KAlF₄.The flux may be prepared by dissolving aluminum hydroxide inhydrofluoric acid having a concentration of 5% to 40% by weight to givean Al:F ratio of 1:4 to 1:4.5 and neutralizing this mixture withpotassium carbonate or hydroxide to give a Al:K ratio of 1:1-1.5.

The addition of magnesium to aluminum to form aluminum magnesium alloysimproves the formability, corrosion resistance and tensile strength ofparts made therefrom. On the other hand, brazing of such alloys withpotassium fluoroaluminate based fluxes becomes more and more difficultwith increased Mg content of the alloy. It was already known to addcesium fluoroaluminate to fluoroaluminate based fluxes to improve thebrazing properties of the flux in view of aluminum magnesium alloys.See, for example, CA 1,239,525.

Japanese patent application JP-S6199569 discloses a method for brazingof aluminum using a flux which contains, on one hand, 73.6% by weight ofKAlF₄ and 0.2 to 18.4% by weight of KF, and, on the other hand, 0.1 to8.0% by weight, of one or more additives of the group consisting of LiF,NaF and CaF₂. According to the examples, the content of KF in the fluxused is from 9.8 to 11% by weight.

Nevertheless, the process and the fluxes for brazing of parts made fromaluminum magnesium alloys are still open for improvements.

According to one aspect, the invention relates to a process for brazingof parts of aluminum alloy comprising equal to or more than 0.5% byweight of magnesium in the interface, comprising a step of applying abrazing flux to at least one of parts to be joined, a step of assemblingthe parts to be joined and a step of heating the parts to be joined to atemperature of equal to or higher than 450° C. to provide brazed jointparts, wherein the brazing flux comprises equal to or more than 65% byweight, preferably equal to or more than 80% by weight, relative to thetotal weight of the brazing flux, of a first component and equal to ormore than 3% by weight of a second component, relative to the totalweight of the brazing flux, with the proviso that the first componentand the second component are not identical, and wherein the firstcomponent is selected from monoalkali tetrafluoroaluminates selectedfrom KAlF₄, CsAlF₄ and mixtures thereof, and the second component is atleast one alkaline or alkaline earth metal compound selected from thegroup consisting of KAlF₄, CsAlF₄, Li₃AlF₆, CaF₂, CaCO₃, MgF₂, MgCO₃,SrF₂, SrCO₃, BaF₂, BaCO₃, and mixtures of two or more thereof;preferably with the proviso that, if at least one compound selected fromthe group consisting of LiF, NaF and CaF₂ are comprised, the content offree KF is lower than 0.1% by weight, relative to the total weight ofthe brazing flux.

The proviso that, if at least one compound selected from the groupconsisting of LiF, NaF and CaF₂ are comprised, the content of free KF islower than 0.1% by weight, relative to the total weight of the brazingflux, relates to the presence of effective amounts; preferably, theproviso relates to the presence of at least one compound selected fromthe group consisting of LiF, NaF and CaF₂ if the sum of all compounds ofthis group is equal to or greater than 0.1% by weight, more preferably,lower than 0.05% by weight. “The term “free”, e.g. in connection withfree KF, denotes the binary salt, e.g. KF which is present as such, thusis not complexed, e.g. in the form of potassium fluoroaluminates.Likewise, the term “free” also denotes binary LiF, CaF₂ or NaF, which isnot complexed. In a preferred embodiment, the content of free KF islower than 0.1% by weight, and more preferably, lower than 0.05% byweight, relative to the total weight of the flux, in all embodiments. Ina still more preferred embodiment, the sum of the content of free KF andof free or complexed NaF is lower than 0.1% by weight, more preferably,lower than 0.5% by weight, in all embodiments. In another preferredembodiment, the content of free KF is lower than 0.1% by weight,relative to the total weight of the brazing flux if at least onecompound selected from the group consisting of LiF, NaF and CaF₂ arecomprised in the flux. Generally, in this embodiment, the at least onecompound selected from the group consisting of LiF, NaF and CaF₂ is notcomplexed, also denoted as free LiF, NaF or CaF₂.

In certain other embodiments, the sum of the content of free KF, free orcomplexed NaF and free LiF is lower than 0.1% by weight, morepreferably, lower than 0.05% by weight, relative to the total content ofthe flux. In another embodiment, the flux is essentially free, andpreferably, free, of LiF, NaF and CaF₂.

Preferably, the brazing flux comprises equal to or more than 80% byweight, relative to the total weight of the brazing flux, of CsAlF4, or,equal to or more than 80% by weight of KAlF₄, relative to the totalweight of the brazing flux, or both KAlF₄ and CsAlF₄, the sum of whichis equal to or more than 80% by weight.

The term “at the interface” is now explained. If two parts of Al orAl—Mg alloy are assembled to be brazed, the areas forming a joint arecalled “interface”. The Mg percentage of both parts is added, and theresult of the addition provides the percentage of Mg at the interface.For example, if two parts with 0.5% by weight of Mg are assembled, thecontent of Mg at the interface is 1% by weight. If a part comprising 1%by weight of Mg is assembled with a part containing no Mg, the Mgcontent at the interface is also 1% by weight.

The upper limit of the Mg content at the interface depends from thebrazing conditions. For example, if flame brazing (i.e. providing heatwith an open flame) is used to braze the parts, the Mg content may be upto 2% by weight at the interface, and even higher. If brazing isperformed according to the Controlled Atmosphere Brazing, i.e. heat isprovided by electrical installations and the rise of temperature of theparts is slower than when a flame is used, the Mg content at theinterface is preferably equal to or lower than 1.5% by weight, morepreferably, equal to or lower than 1.4% by weight.

The brazing temperature is dependent on the nature of the flux. If thefirst component is predominantly or completely constituted by CsAlF₄,the brazing temperature may be as low as 450° C. If the first componentis predominantly or completely constituted by KAlF₄, the brazingtemperature preferably is equal to or greater than 560° C. Here, theupper limit is often 610 to 625° C.

In a preferred embodiment, the first component consists essentially ofKAlF₄.

Preferably, the second component is selected from the group consistingof CsAlF₄, Li₃AlF₆, CaF₂, MgF₂, SrF₂, BaF₂, and mixtures of two or morethereof.

If a Mg containing compound is present, the flux preferably containsless than 1% by weight of the sum of Mg containing compounds such asMgF₂ or MgCO₃.

More preferably, the second component is selected from the groupconsisting of CsAlF₄, Li₃AlF₆, BaF₂, and mixtures of two or morethereof. Especially preferably, the second component comprises CsAlF₄and Li₃AlF₆, and optionally also BaF₂.

In certain embodiments, the sum of the content of free KF, free orcomplexed NaF, free LiF and CaF₂ is lower than 0.1% by weight, morepreferably, lower than 0.05% by weight, relative to the total content ofthe flux.

Preferably, the second component is present in an amount of equal to orgreater than 2% by weight, more preferably, in an amount of equal to orgreater than 3% by weight, and often, in an amount of equal to orgreater than 5% by weight.

Preferably, the second component is present in an amount of equal to orless than 30% by weight, more preferably, in an amount of equal to orless than 20% by weight. If two or more of said components mentionedabove belonging to the group of second components is present, theirtotal amount is equal to or less than 30% by weight.

If present, CaF₂ or CaCO₃ is preferably contained in an amount of equalto or more than 5% by weight. If present, MgF₂ or MgCO₃ is preferablycontained in an amount of equal to or more than 5% by weight. Ifpresent, SrF₂ or SrCO₃ is preferably contained in an amount of equal toor more than 5% by weight. If present, BaF₂ or BaCO₃ is preferablycontained in an amount of equal to or more than 5% by weight. Ifmixtures of fluorides and/or carbonates are present, then their totalcontent is preferably equal to or more than 5% by weight.

If present, CsAlF₄ is preferably present in an amount of equal to ormore than 1% by weight.

If present, Li₃AlF₆ is preferably present in an amount of equal to orgreater than 1% by weight.

If present, CaF₂ or CaCO₃ is preferably contained in an amount of equalto or less than 20% by weight. If present, MgF₂ or MgCO₃ is preferablycontained in an amount of equal to or less than 30% by weight. Ifpresent, SrF₂ or SrCO₃ is preferably contained in an amount of equal toor more than 30% by weight. If present, BaF₂ or BaCO₃ is preferablycontained in an amount of equal to or less than 30% by weight. Ifmixtures of fluorides and/or carbonates are present, then the sum oftheir content is preferably equal to or less than 30% by weight.

If present, CsAlF₄ is preferably present in an amount of equal to orless than 10% by weight, more preferably, equal to or less than 5% byweight.

If present, Li₃AlF₆ is preferably present in an amount of equal to orless than 8% by weight.

Especially preferred ranges are: CaF₂ or MgF₂ in an amount of from 5 to20% by weight, SrF₂ or BaF₂ in an amount of from 10 to 20% by weight; acombination of SrF₂ or BaF₂ in a total amount of from 10 to 20% byweight; CsAlF₄ in an amount of from 1 to 5% by weight; Li₃AlF₆ in anamount of from 1 to 8% by weight; CsAlF₄ in an amount of from 1 to 5% byweight and Li₃AlF₆ in an amount of from 1 to 8% by weight.

More preferably, the second component is selected from the groupconsisting of CsAlF₄, Li₃AlF₆, BaF₂, and mixtures of two or morethereof; and still more preferably, the second component is acomposition consisting of CsAlF₄ and Li₃AlF₆; and optionallyadditionally BaF₂.

Highly preferred brazing fluxes are used which comprise, or consist of,KAlF₄ in a range of from equal to or greater than 80 to equal to orlower than 98% by weight, CsAlF₄ in a range of from equal to or morethan 1% by weight to equal to or less than 10% by weight, Li₃AlF₆ in arange of from equal to or more than 1% by weight to equal to or lessthan 10% by weight, and BaF₂ in a range of from 0 to equal to or lessthan 15% by weight.

Preferred brazing fluxes are given in tables 1 to 4.

TABLE 1 Preferred brazing fluxes for use in Al—Mg alloy brazing KAlF₄AEF₂ Li₃AlF₆ CsAlF₄ [% by weight] [% by weight] [% by weight] [% byweight] 80 CaF₂ 20 0 0 80 MgF₂ 20 0 0 80 SrF₂ 20 0 0 80 BaF₂ 20 0 0 80SrF₂ 10 0 0 BaF₂ 10 85 SrF₂ 15 0 0 85 BaF₂ 15 0 0 85 SrF₂ 5 0 0 SrF₂ 1082 CaF₂ 15 0 3 82 SrF₂ 15 0 3 82 MgF₂ 15 0 3 82 BaF₂ 15 0 3 82 SrF₂ 7.50 3 BaF₂ 7.5 80 CaF₂ 12 5 3 80 MgF₂ 12 5 3 80 SrF₂ 12 5 3 80 BaF₂ 12 5 380 SrF₂ 6 5 3 BaF₂ 6 90 CaF₂ 10 0 0 90 MgF₂ 10 0 0 90 SrF₂ 10 0 0 90BaF₂ 10 0 0 90 SrF₂ 5 0 0 BaF₂ 5 94 — 2 4 93 — 4 3 84 BaF₂ 10 2 4 83BaF₂ 10 4 3 “AEF₂” denotes alkaline earth metal fluoride.

If desired, KAlF₄ can be introduced in the form of commerciallyavailable mixtures with K₂AlF₅ or its hydrates. Such a mixture whichcomprises approximately 80% by weight of KAlF₄ and approximately 20% byweight of K₂AlF₅ or its hydrates, is available from Solvay Fluor GmbHunder the tradename Nocolok®. Preferred mixtures of this kind are givenin table 2.

TABLE 2 Preferred brazing fluxes for use in Al—Mg alloy brazing with amixture of KAlF₄ and K₂AlF₅ (4:1 w/w). KAlF₄/K₂AlF₅ mixture AEF₂ CsAlF₄Li₃AlF₆ [% weight] [% weight] [% weight] [% weight] 85 CaF₂ 15 — — 85MgF₂ 15 — — 85 SrF₂ 15 — — 85 BaF₂ 15 — — 90 CaF₂ 10 — — 90 SrF₂ 10 — —90 BaF₂ 10 — — 84 BaF₂ 10 2 4 93 — 4 3 94 — 2 4 83 BaF₂ 10 4 3 60 KAlF₄30 — — BaF₂ 10 — —

If desired, KAlF₄ and Li₃AlF₆ can be introduced in the form ofcommercially available mixtures with K₂AlF₅ or its hydrates. Such amixture which comprises approximately 77% by weight of KAlF₄,approximately 19% by weight of K₂AlF₅ or its hydrates, and approximately4% by weight of Li₃AlF₆ is available from Solvay Fluor GmbH under thetradename Nocolok®-Li. Preferred mixtures of this kind are given intable 3.

TABLE 3 Preferred brazing fluxes for use in Al—Mg alloy brazing with amixture of KAlF₄, K₂AlF₅ and Li₃AlF₆ (77:19:4 w/w/w)KAlF₄/K₂AlF₅/Li₃AlF₆ mixture AEF₂ CsAlF₄ [% by weight] [% by weight] [%by weight] 90 CaF₂ 10 — 90 MgF₂ 10 — 90 SrF₂ 10 — 90 BaF₂ 10 — 87 CaF₂10 3 87 MgF₂ 10 3 87 SrF₂ 10 3 87 BaF₂ 10 3

If desired, KAlF₄ and CsAlF₄ can be introduced in the form ofcommercially available mixtures with K₂AlF₅ or its hydrates. Such amixture which comprises approximately 79% by weight of KAlF₄,approximately 19% by weight of K₂AlF₅ or its hydrates, and approximately2% by weight of CsAlF₄ is available from Solvay Fluor GmbH under thetradename Nocolok®-Cs. Preferred mixtures of this kind are given intable 4.

TABLE 4 Preferred brazing fluxes for use in Al—Mg alloy brazing with amixture of KAlF₄, K₂AlF₅ and CsAlF₄ (79:19:2 w/w/w) KAlF₄/K₂AlF₅/CsAlF₄mixture AEF₂ Li₃AlF₆ [% by weight] [% by weight] [% by weight] 90 CaF₂10 — 90 SrF₂ 10 — 90 BaF₂ 10 — 87 BaF₂ 10 3

According to one preferred embodiment, the first component is KAlF₄ inan amount of equal to or greater than 80% by weight relative to thetotal weight of the brazing flux, and the second component is present inan amount of equal to or greater than 5% by weight, relative to thetotal weight of the brazing flux, and is selected from the groupconsisting of CsAlF, Li₃AlF₆ and a mixture of both; additionally, CaF₂,MgF₂, SrF₂, BaF₂, and mixtures of two or more thereof may be present.

If the flux does not consist of said first and second components, thebalance to 100% by weight are constituted from other fluxes known in theart, preferably, K₂AlF₅ or K₂AlF₅.H₂O. Preferably, the content of K₃AlF₆is lower than 1% by weight, including 0% by weight.

It is preferred that KAlF₄ is essentially the only potassiumfluoroaluminate in the brazing flux. The total content of K₂AlF₅, itshydrate and of K₃AlF₆, if present, is preferably equal to or lower than5% by weight, relative to the total weight of the brazing flux.

According to one embodiment, the brazing flux consists of said first andsecond components.

The brazing process can be performed applying the brazing flux accordingto the dry fluxing method. The flux may, for example, be provided to thesurface of the items to be brazed by pneumatic transport and adheresmechanically to the surface of the items to be brazed.

If the brazing flux is not applied according to the dry fluxing method,it is applied according to the wet fluxing method. In the wet fluxingmethod, a brazing flux composition is applied which contains the fluxand additives; liquid carriers like water or alcohols are considered asadditives in the present invention.

The brazing flux composition can, for example, be sprayed onto at leastone of the parts to be joined, it can be painted on their surface withbrushes, or it can be applied by immersing parts into liquidcomposition.

According to one embodiment, the brazing flux composition of the presentinvention is applied according to the wet fluxing method and containsthe brazing flux suspended in water, water-free organic liquids oraqueous organic liquids. Preferred liquids are those that have a boilingpoint at ambient pressure (1 bar abs) of equal to or lower than 350° C.The term “suspended in water” does not exclude that a part of the fluxcomposition is dissolved in the liquid; this may be the case especiallywhen water or aqueous organic liquids are contained. Liquids that arepreferred are deionized water, mono-, di- or tribasic aliphaticalcohols, especially those with 1 to 4 carbon atoms, e.g. methanol,ethanol, isopropanol, or ethylene glycol, or glycol alkyl ethers,wherein alkyl preferably denotes linear aliphatic C1 to C4 alkyl orbranched C3 to C4 alkyl. Non-limiting examples are glycol monoalkylethers, e.g. 2-methoxyethanol or diethylene glycol, or glycol dialkylethers, for example, dimethyl glycol (dimethoxyethane). Mixturescomprising two or more of the liquids are also suited very well.Isopropanol or mixtures containing isopropanol are especially suitable.

In a preferred embodiment, the composition comprising the brazing fluxand a liquid also contains further additives which improve the brazingflux composition properties.

In an especially preferred embodiment the brazing flux is present in theform of a flux composition wherein the flux is suspended in a liquidwhich also contains a binder. Binders improve, for example, the adhesionof the brazing flux after its application on the parts to be brazed.Thus, the wet flux method using a brazing flux composition comprisingthe brazing flux, binder and water, organic liquid or aqueous organicliquid is a preferred embodiment of the brazing process of the presentinvention. The liquid serves as solvent for some additives or as acarrier to disperse the flux or other insoluble components of thecomposition.

Suitable binders can be selected for example from the group consistingof organic polymers. These binders form a coating on aluminum parts oraluminum alloy parts when they are physically dried (i.e., they form asolid coating after the liquid is removed by evaporation), or when theyare chemically dried (they form a solid coating e.g. under the influenceof chemicals, e.g. oxygen or light which causes a cross linking of themolecules, or by a thermal treatment which provokes cross linking). Bothmechanisms may occur simultaneously. Suitable polymers include polymerswhich are present in the composition in the form of a dispersion ofsolid particles dispersed in the liquid, and polymers which are presentin the form of a solution dissolved in the liquid. Highly suitablebinders are polyolefines, e.g. butyl rubbers, polyurethanes, resins,phthalates, polyacrylates, polymethacrylates, vinyl resins, epoxyresins, nitrocellulose, polyvinyl acetates or polyvinyl alcohols.Brazing flux compositions containing water as a liquid and water-solublepolymer or a polymer dispersed in water, for example, polyvinyl alcoholor polyurethane, are especially suitable because they have the advantagethat, during the brazing process, water is evaporated instead ofpossibly flammable organic liquids.

The compositions may include other additives which improve theproperties of the composition, for example, suspension stabilizers,surfactants, especially nonionic surfactants, e.g. Antarox BL 225, amixture of linear C8 to C10 ethoxylated and propoxylated alcohols,thickeners, e.g. methyl butyl ether, thixotropic agents, e.g. gelatineor pectines, or a wax as described in EP-A 1808264.

The content of the brazing flux in the total brazing flux composition(including, for example, liquid or liquids, thixotropic agents,surfactants and binders, if present) generally is equal to or greaterthan 0.75% by weight. Preferably, the content of the brazing flux in theflux composition is equal to or greater than 1% by weight. Morepreferably, the brazing flux content in the brazing flux composition isequal to or greater than 5% by weight, very preferably, equal to orgreater than 10% by weight of the total flux composition.

Generally, the brazing flux content in the brazing flux composition isequal to or lower than 70% by weight. Preferably, it is equal to orlower than 50% by weight.

The binder, if present, is generally contained in an amount of equal toor greater than 0.1% by weight, preferably equal to or greater than 1%by weight of the total brazing flux composition. The binder, if present,is generally contained in an amount equal to or lower than 30% byweight, preferably equal to or lower than 25% by weight of the totalbrazing flux composition.

The thixotropic agent, if present, is generally contained in an amountof equal to or greater than 1% by weight of the total brazing fluxcomposition. Generally, if present, it is contained in an amount equalto or lower than 20% by weight, preferably in an amount equal to orlower than 10% by weight.

The thickener, if present, is generally contained in an amount of equalto or greater than 1% by weight, preferably equal to or greater than 5%by weight of the total brazing flux composition. Generally, thethickener, if present, is contained in an amount equal to or lower than15% by weight, preferably equal to or lower than 10% by weight, of thetotal brazing flux composition.

Highly suitable brazing flux compositions for wet applications contain10 to 70% by weight of the brazing flux, 1 to 25% by weight binder, 0 to15% by weight of a thickener, 0 to 10% by weight of a thixotropic agent,and 0 to 5% by weight of other additives, e.g. a surfactant or asuspension stabilizer. Preferably, the remainder to 100% by weight iswater, an organic solvent or an aqueous organic solvent; water isespecially preferred.

In another embodiment, the flux composition is free of any water orwater-free or aqueous organic liquid, but contains the flux (and, ifdesired, additives, e.g. surfactants, thickener or thixotropic agents)as described above, and a water-soluble organic polymer as a binderwhich is present in the form of a water soluble package for the flux.For example, polyvinyl alcohol is very suitable as water-soluble packagefor the flux as described in US patent application publication2006/0231162. Such packages can be handled without dust formation, andafter addition of water or aqueous organic solvents, they form asuspension of the flux in water while the water soluble polymerdissolves and provides the binder function.

The suspension can be applied in a known manner, e.g. by spraying,painting, or by dipping the parts to be joined into the suspension.

If desired, parts coated with the flux composition can be dried beforebrazing and then later be brazed, or drying and brazing can be performedimmediately after one another.

The components can be applied separately to the parts or in the form ofa premixed brazing flux.

The weight per area of the brazing flux applied on the parts to bebrazed is preferably equal to or higher than 4 g/m². More preferably, itis equal to or higher than 5 g/m². Preferably, it is equal to or lowerthan 50 g/m², especially preferably equal to or lower than 20 g/m². If abrazing flux composition is applied, the flux load is correspondinglyhigher to achieve the brazing flux load as mentioned above. If, to givean example for calculation, a brazing flux composition comprising 50% byweight of the brazing flux is applied, then the load of the brazing fluxcomposition is preferably equal to or higher than 8 g/m² to achieve abrazing flux load of equal to or higher than 4 g/m².

The heat needed for brazing the parts may be provided by an open flame,but inductive heating or heating by means of a laser is also possible.It is preferred to braze according to the Controlled Atmosphere Brazing(“CAB”) process in the presence of N₂ or Ar gas.

Generally, a solder metal is applied. Often, solders consisting of Aland Si are applied. It is preferred to braze parts which are clad with asolder.

The process is generally performed to braze parts of Al—Mg alloys (whichmay contain further metals) with parts of Al—Mg alloys (which maycontain further metals) or with Al parts (which may contain other metalsthan Mg). The content of Mg in the parts depends on the desired fluxloading and of the sum of the magnesium content of the alloys at theinterface of the parts to be joined. For example, if a flux loading inthe upper range, e.g. from 10 to 20 g/m² is technically acceptable, aninterface comprising equal to or more than 0% by weight to equal to orless then 1.8% by weight of Mg, preferably equal to or more than 0.8% byweight to equal to or less then 1.5% by weight of Mg can preferably bebrazed.

If a flux loading in the lower range, e.g. from 4 to 10 g/m² is desired,an interface comprising equal to or more than 0% by weight to equal toor less then 1.0% by weight of Mg, preferably equal to or more than 0.6%by weight to equal to or less then 1.0% by weight of Mg can preferablybe brazed.

The Mg content at the interface is calculated by adding the sum of Mgpercentages in both parts to be joined. If, for example, two parts eachwith 0.68% by weight of Mg are to be joined, the Mg content at theinterface is (0.68+0.68=) 1.36% by weight. If a part with 0.68% byweight of Mg and a part with 0% by weight of Mg are brazed, the Mgcontent at the interface is (0.68+0=) 0.68% by weight.

Brazing of parts with a Mg content in the interface in a range of from0.6 to 1% by weight and a brazing flux load in a range of from 4 to 10g/m² is preferred.

For example, the following kind of parts can be brazed: Heat exchangers(heater cores for oil coolers/water coolers, radiators, air conditionersand parts thereof, e.g. condensers, evaporators), sheets, thin sheets,fin to tube, endplates (used as core reinforcement) brazed to fin,headers to tube, fin to headers, tube to tube, folded tubes, extrudedtubes, and fittings (e.g. nuts and bolts). Another aspect of the presentinvention concerns a brazing flux which may be used in the abovedescribed process for brazing of Al—Mg alloys.

The invention also relates to a brazing flux which is suitable to beused in the process of the invention mentioned above. The claimedbrazing flux is a selection of the flux explained above in thedescription of the process of the invention.

Preferably, the brazing flux has a content of KAlF₄ which is equal to orgreater than 80% by weight, relative to the total weight of the flux.

The brazing flux of the invention comprises, relative to the totalweight of the brazing flux, equal to or more than 80% by weight of afirst component selected from the group consisting of KAlF₄ and CsAlF₄,and a second component selected from the group consisting of Li₃AlF₆,CaF₂, CaCO₃, MgF₂, MgCO₃, SrF₂, SrCO₃, BaF₂, BaCO₃, and mixtures of twoor more of said second components; or it consists of said first andsecond components.

Preferably, the brazing flux of the invention also is a selection inthat the claimed flux mandatorily contains equal to or more than 1% byweight of CsAlF₄. Preferably, the flux comprises, relative to the totalweight of the brazing flux, equal to or more than 80% by weight ofKAlF₄, equal to or more than 1% by weight of CsAlF₄, and equal to ormore than 2% by weight of at least one component selected from the groupconsisting of Li₃AlF₆, CaF₂, CaCO₃, MgF₂, MgCO₃, SrF₂, SrCO₃, BaF₂,BaCO₃, and mixtures of two or more of said second components; or itconsists of said components.

More preferably, the brazing flux of the invention comprises, orconsists of, relative to the total weight of the brazing flux, equal toor more than 80% by weight of KAlF₄, equal to or more than 1% by weightof CsAlF₄, and equal to or more than 3% by weight of at least onecomponent selected from the group consisting of Li₃AlF₆, CaF₂, CaCO₃,MgF₂, MgCO₃, SrF₂, SrCO₃, BaF₂, BaCO₃, and mixtures of two or more ofthese latter components.

Preferably, the brazing flux of the invention comprises, relative to thetotal weight of the brazing flux, equal to or more than 80% by weight ofKAlF₄, and equal to or more than 1% by weight of CsAlF₄, and equal to ormore than 2% by weight of a second component selected from the groupconsisting of Li₃AlF₆, CaF₂, CaCO₃, SrF₂, SrCO₃, BaF₂, BaCO₃, andmixtures of two or more of said second components; or it consists ofKAlF₄, CsAlF₄ and one or more of said second components.

If the flux does not consist of said components, the balance to 100% byweight are other flux components known in the art, preferably, K₂AlF₅ orK₂AlF₅.H₂O. Preferably, the content of K₃AlF₆ is lower than 1% byweight, including 0% by weight.

In certain embodiments, the sum of the content of free KF, free orcomplexed NaF and free LiF is lower than 0.1% by weight, morepreferably, lower than 0.05% by weight, relative to the total content ofthe flux of the invention. In another embodiment, the flux of theinvention is essentially free, and preferably, free, of LiF, NaF andCaF₂.

Preferably, the second component is selected from the group consistingof Li₃AlF₆, CaF₂, MgF₂, SrF₂, BaF₂, and mixtures of two or more of saidsecond components.

A preferred embodiment of the brazing flux comprises or consists of,relative to the total weight of the brazing flux, equal to or more than80% by weight of KAlF₄, equal to or more than 1% by weight of CsAlF₄;and equal to or more than 3% by weight of a second component selectedfrom the group consisting of Li₃AlF₆, CaF₂, MgF₂, SrF₂, BaF₂, andmixtures of two or more of said second components.

According to one embodiment, the brazing flux consists essentially ofKAlF₄, CsAlF₄, and at least one second component selected from Li₃AlF₆,SrF₂, BaF₂, and mixtures of 2 or more thereof. The term “essentially” inthe present invention preferably denotes a content of equal to or lessthan 5% by weight of other salts like K₂AlF₅, its hydrates, and K₃AlF₆.

A very preferred brazing flux consists essentially of KAlF₄, CsAlF₄,Li₃AlF₆ and optionally includes one or more of CaF₂, MgF₂, SrF₂, andBaF₂.

A still more preferred brazing flux consists essentially of KAlF₄,CsAlF₄, and Li₃AlF₆ and optionally includes BaF₂.

Highly preferred brazing fluxes comprise, or consist of, KAlF₄ in arange of from equal to or greater than 80 to equal to or lower than 98%by weight, CsAlF₄ in a range of from equal to or more than 1% by weightto equal to or less than 10% by weight, Li₃AlF₆ in a range of from equalto or more than 1% by weight to equal to or less than 10% by weight, andBaF₂ in a range of from 0 to equal to or less than 15% by weight.

Still more preferred brazing fluxes comprise, or consist of, KAlF₄ in arange of from equal to or greater than 80 to equal to or lower than 98%by weight, CsAlF₄ in a range of from equal to or more than 1% by weightto equal to or less than 5% by weight, Li₃AlF₆ in a range of from equalto or more than 1% by weight to equal to or less than 8% by weight, andBaF₂ in a range of from 0 to equal to or less than 12% by weight.

The total content of K₂AlF₅ and its hydrates and of K₃AlF₆ in thebrazing flux is preferably equal to or lower than 4% by weight, morepreferably, equal to or lower than 3% by weight.

The brazing fluxes described above can be manufactured by mixing theseparate components, for example, by mixing KAlF₄, Li₃AlF₆, CsAlF₄ andBaF₂, or by co-precipitation. For example, HalF₄ is reacted with KOH,CsOH and Ba(OH)₂ to provide a brazing flux comprising potassium cesiumtetrafluoroaluminate and BaF₂. Here, the content of K⁺ and Cs⁺ is suchthat a neutral complex with AlF₄ ⁻ is formed.

Still another embodiment of the present invention concerns a brazingflux composition which comprises the brazing flux as described above andat least one brazing additive. Preferred brazing additives are describedabove, for example, liquids, binders, suspension stabilizers,surfactants, thickeners, and thixotropic agents.

Preferred features of the brazing flux and of the brazing fluxcomposition correspond to those features described above as beingpreferred embodiments of the fluxes and compositions.

Still another aspect of the present invention are coated parts ofaluminum (which may contain other metals except Mg) oraluminum-magnesium alloys (which may contain other metals), coated witha flux as described above, or coated with a brazing flux composition asdescribed above. The brazing flux load preferably is equal to or greaterthan 4 g/m²; preferably, the flux load is equal to or lower than 15g/m². Parts with a flux load in the range of from 5 to 10 g/m² areespecially preferred. If the brazing flux is contained in a brazing fluxcomposition, the load of the brazing flux composition is respectivelyhigher as explained above. Preferred coated parts are: Heat exchangers(and parts thereof, e.g. heater cores for oil coolers/water coolers,radiators, air conditioners and parts thereof, e.g. condensers,evaporators), sheets, thin sheets, fins and tubes, endplates (used ascore reinforcement) to be brazed to fins, headers to be brazed to atube, fins to be brazed to headers, tubes to be brazed to a tube, foldedtubes, extruded tubes, and fittings (e.g. nuts and bolts). For example,thin Al—Mg alloy sheets (fins) with a thickness of equal to or lowerthan 0.1 mm down to 0.06 mm can be brazed, as well as tubes with athickness of equal to or lower than 0.5 mm down to 0.25 mm.

Another aspect of the present invention concerns brazed parts, obtainedby brazing respective parts using the inventive brazing flux, obtainedby brazing coated parts according to the invention, or by brazing partsaccording to the process of the present invention.

The advantage of the brazing process, the brazing flux and the brazingflux composition described above is especially that they allow brazingof Al—Mg alloys with a high amount of Mg, especially with equal to ormore than 0.5% by weight of Mg at the interface, and up to 2% by weightand even more. Additionally, many of the brazing fluxes have a lowercorrosion potential than known fluxes.

Should the disclosure of any patents, patent applications, andpublications which are incorporated herein by reference conflict withthe description of the present application to the extent that it mayrender a term unclear, the present description shall take precedence.

The examples which follow are intended to illustrate the presentinvention without, however, limiting the scope thereof.

EXAMPLES General Remarks: Used Aluminum Parts:

-   a) Commercially available AMAG 6951 brazing sheet (0.68% Mg, 4343    clad) and clad-less AMAG angle material (0.68% Mg) from Austria    Metal AG were used. The Mg content in the metal-to-metal interface    adds up to 1.36% by weight Mg (2×0.68%) in total.-   b) Commercially available AMAG 6951 brazing sheet (0.68% Mg, 4343    clad) and clad-less AMAG AA1050 (Al 99.5%) angle from Austria Metal    AG were used. The Mg content in the metal-to-metal interface adds up    to 0.68% by weight Mg (0.68+0%) in total.

Brazing Process:

Brazing was performed according to a standard CAB brazing profile and 25by 25 mm clad sheet coupons (single side) with angle on top. Fluxing wasdone manually (flux load weight on precision scale, drops of isopropanoland homogenous spreading). Each test was performed 3 times.

Evaluation of Joints:

After brazing, the angle was removed by pulling, and the interfacesection was analyzed by Scanning Electron Microscopy coupled with EnergyDispersive X-ray (SEM/EDX) Spectroscopy.

Example 1: AMAG 6951 Brazing Sheet (0.68% Mg, 4343 Clad) and Clad-LessAMAG Angle Material (0.68% Mg)

The flux load in examples 1.X was always 10 g/m².

Example 1.1 (Comparison): Brazing with Nocolok® Cs

Nocolok®Cs was used as flux. SEM/EDX showed a very thin seam with a lowand in part no meniscus.

Example 1.2.: Brazing with KAlF4 and BaF2

A mixture of KAlF₄ (90% by weight) and BaF₂ (10% by weight) was used asflux. SEM/EDX showed a very thin seam with a small meniscus.

Accordingly, the joint was slightly better with this flux.

Example 1.3: Brazing with KAlF4/CsAlF4/Li3AlF6

A mixture of KAlF₄ (94% by weight), CsAlF₄ (2% by weight) and Li₃AlF₆(4% by weight) was used as a flux. The brazing was performed 3 times.Two times, a good brazing result was achieved.

Example 1.4: Brazing with KAlF4/CsAlF4/Li3AlF6

A mixture of KAlF₄ (93% by weight), CsAlF₄ (4% by weight) and Li₃AlF₆(3% by weight) was used as a flux. The brazing was performed 3 times.The seam was thin, the meniscus was small.

Examples 2.X: The Flux Load in Examples 1.X was Always 15 g/m2

Examples 1.1 to 1.4 were repeated, but with said 15 g/m² flux load.

All specimens showed a small meniscus. Examples 2.2 to 2.4 (according tothe invention) always showed a better brazing result than comparisonexample 2.1.

Example 3: Brazing with AMAG 6951 Brazing Sheet (0.68% Mg, 4343 Clad)and Clad-Less AMAG AA1050 (Al 99.5%) Angle

The flux load always was 5 g/m².

Example 3.1 (Comparison): Brazing with Nocolok® Cs

Brazing was performed using Nocolok®Cs. After brazing, a partial thinmeniscus was observed.

Example 3.2: Brazing with KAlF4, CsAlF4 and Li3AlF6 (4% by Weight)

A mixture of KAlF₄ (94% by weight), CsAlF₄ (2% by weight) and Li₃AlF₆(4% by weight) was used as a flux. The brazing was performed 3 times.All three times, a well-developed meniscus was observed.

The examples above demonstrate that the fluxes according to theinvention are superior to Nocolok®Cs, a flux recommended for brazing ofAl—Mg alloys, and which comprises approximately 79% by weight of KAlF₄,approximately 19% by weight of K₂AlF₅ or its hydrates, and approximately2% by weight of CsAlF₄.

Example 4: Flame Brazing

A brazing sheet and an angle, both from Al—Mg alloy with 0.75% by weightof Mg, clad with Al—Si12 alloy, are assembled and heated with a torchflame until flux and then the cladding melts to provide a brazedassembly.

1.-10. (canceled)
 11. A brazing flux comprising, relative to the totalweight of the brazing flux, equal to or more than 80% by weight of KAlF₄and equal to or more than 1% by weight of CsAlF₄, and equal to or morethan 2% by weight of a second component selected from the groupconsisting of Li₃AlF₆, CaF₂, CaCO₃, MgF₂, MgCO₃, SrF₂, SrCO₃, BaF₂,BaCO₃, and mixtures of two or more of said second components.
 12. Thebrazing flux of claim 11 comprising, relative to the total weight of thebrazing flux, equal to or more than 80% by weight of KAlF₄, equal to ormore than 1% by weight of CsAlF₄, and equal to or more than 3% by weightof a second component selected from the group consisting of Li₃AlF₆,CaF₂, MgF₂, SrF₂, BaF₂, and mixtures of two or more of said secondcomponents.
 13. The brazing flux of claim 11 comprising or consistingof, relative to the total weight of the brazing flux, equal to or morethan 80% by weight of KAlF₄, equal to or more than 1% by weight ofCsAlF₄, and wherein the second component is selected from the groupconsisting of Li₃AlF₆, BaF₂, and mixtures thereof.
 14. The brazing fluxof claim 11 consisting essentially of KAlF₄, CsAlF₄ and Li₃AlF₆ andoptionally BaF₂.
 15. A brazing flux composition comprising a brazingflux according to claim 11 and at least one brazing additive.
 16. Coatedparts of aluminum or aluminum-magnesium alloys, coated with a fluxaccording to claim
 11. 17. Coated parts of aluminum oraluminum-magnesium alloys, coated with a brazing flux compositionaccording to claim
 15. 18. The brazing flux according to claim 11,wherein a content of free KF is lower than 0.1% by weight, relative tothe total weight of the brazing flux if at least one compound selectedfrom the group consisting of LiF, NaF and CaF₂ is comprised in the flux.